This invention relates to a method and system of driving a pair of stereo loudspeakers, or a pair of loudspeakers that are part of a multiple loudspeaker array, with a non-zero optimal phase relationship in the low frequency region, approximating 90 degrees in the region where localization becomes impossible. In particular, this method and system allows room modes to be excited independently by both loudspeakers thereby producing a full bass response which may be further modified by a shelf filter to equalize the sound pressure level in the bass frequency range.
Stereophonic pairs of loudspeakers are used for sound reproduction in a listening room, and are often called upon to reproduce sound in the low frequency region where localization becomes impossible. Such loudspeakers are commonly used in pairs or multiples, and frequently are in combination with other loudspeakers intended to reproduce the higher audio frequencies.
In some sound systems, known as satellite-subwoofer systems, the upper frequency range is reproduced by a pair of satellite loudspeakers with a limited response below about 200 Hz, the low frequency signals being combined into a single monophonic subwoofer, often placed along the center line of the room. For such systems, this invention has no application. However, when full-range loudspeakers are used, or when a user installs a pair of subwoofers, the present invention, as described in detail below, can provide better performance than the conventional usage wherein the loudspeakers are driven in phase at low frequencies.
When two speakers are driven in phase, the performance of full-range speakers or dual subwoofers can be inferior to that of the satellite-subwoofer system. The satellite-subwoofer system will give identical results when the subwoofer is located along the center line of the room, but more bass (and a more spacious sound) can be obtained if the subwoofer is placed off axis and near a corner of the room. With two subwoofers driven in phase, the best placement is to put both together in the same corner of the room.
In the conventional methods of sound reproduction, a coherent phase relationship between the two loudspeakers has been considered necessary so that xe2x80x9cpair-wise mixingxe2x80x9d and the use of pan-pots in recording stereophonically will result in correct imaging of the reproduced sounds. There is a large body of literature on the merits of phase coherence in stereophonic reproduction systems, and great pains are taken, not only to match the amplitude and phase response of amplifiers and equalizers, but also to match pairs of loudspeakers.
Nevertheless, much of the literature also asserts that below some low frequency, usually estimated as between 80 Hz and 200 Hz, localization of sounds is not possible. Above this frequency,the amplitude difference between the loudspeakers is the primary localization cue, but along the centerline of a relatively absorptive room the phase relationship also has an effect.
A neglected consequence of these assertions is that there must be some frequency below which the speakers need not be in phase. The question arises of what is the optimal phase relationship below this frequency. Research briefly described herein has indicated that there is a non-zero optimal phase relationship, and that the loudspeakers should ideally have a phase difference of 90 degrees in the low frequency region.
The present invention is concerned with a method and system for driving a pair of loudspeakers in an appropriate phase relationship in the low frequency region, by means of a phase shifting and equalizing circuit.
One embodiment includes a pair of all-pass phase difference networks optimally producing a quadrature relationship in the low frequency region below about 200 Hz, while maintaining an in-phase relationship at higher frequencies, and additionally includes equalizing networks to introduce a small bass boost to compensate for the xcx9c3 dB reduction in bass energy along the center line of the room that the quadrature circuit often produces.
In another aspect, an electronic signal processing system is provided for driving first and second power amplifiers and first and second loudspeaker systems in a listening room. First and second input terminals are adapted to receive a stereophonic pair of audio input signals. Corresponding first and second output terminals produce a stereophonic pair of output signals for driving the first and second power amplifiers and the first and second loudspeaker systems. Circuitry is provided for varying the phase relationship between said first and second output signals such that their phase difference tends towards zero at high audio frequencies and increases to approximately quadrature or 90 degrees phase difference at low frequencies, the gain of the circuitry between each said input terminal and said corresponding output terminal being approximately constant at all audio frequencies, thereby providing increased apparent spaciousness of low frequency sounds reproduced by said loudspeakers in said listening room.
In another aspect, the signal processing system further comprises a circuit between each input terminal and said corresponding output terminal wherein there is an equal bass boost applied to both said first and second output signals of approximately 3 dB for those frequencies where the phase difference is approximately 90 degrees thereby maintaining approximately constant sound pressure at all frequencies along the lateral center line from front to back of said listening room.
In another aspect, the phase difference of approximately 90 degrees at low frequencies is obtained without active electronics, by combining differences in the loudspeakers themselves, such as differences in enclosure volume, port area, cone mass, surround stiffness, and crossover components.
Although research has shown that relatively steady low frequency sounds are difficult to localize, percussive sounds include higher frequency components that are easier to localize. The ability to localize percussive sounds increases as the frequency rises. The addition of a 90 degree phase shift as provided by this invention sometimes alters the localization of all sounds, and in particular percussive sounds. The 90 degree phase shift results in a time delay in the lagging phase channel, which tends to produce localization shifting towards the leading phase channel. Thus the shift in localization will determine the highest frequency at which the 90 degree phase shift should be applied. For reproduction of classical music, where bass instruments are mostly located on the right of the orchestra as heard by the listener, the leading phase shift should be applied to the right channel to minimize the perception of this effect. However, popular music is typically mixed so that the kick drum (or similar instruments) produces equal levels in both channels, which results in a strongly percussive low frequency sound. When such recordings are played in a laterally symmetric room with phase coherent loudspeakers, the result is a strong in-head localization of the low frequency energy. While this sounds unnatural, it is often considered desirable by recording engineers as evidencing good imaging. The present invention, by reducing in-head localization, may make some of these engineers unhappy. In order to make them happy again, an additional aspect of this invention is additional circuitry that detects rapidly rising (percussive) sound, and temporarily reduces the phase shift.
An advantage of the invention is to provide a richer, fuller bass sound by exciting more room modes. It has been shown that in a laterally symmetric room, the use of a single non-central loudspeaker can excite both odd and even lateral room modes, while a centrally placed loudspeaker cannot excite the odd modes, and the same is true of two loudspeakers driven in phase and symmetrically placed. The sense of spaciousness in the sound reproduction is enhanced, particularly in smaller listening rooms, by exciting both the odd and even lateral modes.
Another advantage of the invention is that it reduces unnatural in-head localization of certain low frequency sounds.
A further advantage of the invention is that it provides an optimal phase relationship between the loudspeakers in the low bass region while maintaining the bass response perceived by the listener by enhancing the base response of the loudspeakers.
An additional advantage of the invention is that it enhances the spaciousness perceived by the listener for low frequency sounds, while maintaining the sound pressure level the same as for two subwoofers in the same corner of the room.